A lot of words have been written about the prospect of the USA and the world running short of oil and natural gas. Together, these supply much of the energy used in the USA for transportation, fertilizer and home heating. Word is that North American gas production has already peaked, and world production of oil is predicted to peak as early as this Thanksgiving. (Coal, which supplies half the electricity used in the United States, shows no immediate prospect of shortages.)

Economic expansion depends on greater yield from whatever inputs are available. The energy intensity of the US economy has been falling steadily since the 1970's, yielding more and more economic output per unit of energy; still, greater efficiency can be overcome by falling supply. The alternative is to convert something else to suit. American society is no stranger to this phenomenon; in the past 229 years, US society has seen a number of transformations in its use of energy for various purposes:

Each transformation either created a resource out of a material which had not been used widely or at all before (coal, petroleum, natural gas) or greatly increased the efficiency of use (fluorescent lighting, diesel locomotives vs. oil-fired steam). Each time, some resource was leveraged to yield more benefit.

The USA's energy supplies come mostly from fossil sources (counting nuclear as fossil), with hydropower being by far the biggest renewable contributor at 7% of electric production. The other two readily accessible and renewable energy supplies, wind and solar, contribute relatively little. Surprisingly, per the EIA waste provides twice as much electricity as wind, and wood almost four times as much. Obviously there are resources which are not being leveraged to best advantage.

One of the principles of life is that one organism's waste is some other organism's resource. Our energy systems don't follow this; the vast majority of our energy comes from once-through handling of a single supply with the products dumped to landfills or the atmosphere, and the closest we come is with cogeneration systems which use effluent heat from power production as space heat or industrial process heat. Is it possible to do better? Looking at the electricity production statistics and then at municipal solid waste production, it looks like there are possibilities we might be ignoring.

So what do we have?

Lots and lots, both resources that are being somewhat underused and resources which are almost entirely ignored. In the "somewhat underused" category, we have the fuel which is burned for industrial process heat and space heat; if half of these uses were adapted to cogeneration, the additional power would amount to tens of gigawatts (perhaps 70+ GW, or almost 1/6 of current generation). But these draw from existing fuels which are rising in price and shrinking in availability; boosting efficiency can get us by for a while, but we're eventually going to have to use something else. Replacing a few percent of current inputs (a la ethanol) isn't nearly ambitious enough; ideally, it would be something we can leverage to power most of our industry and transport.

The solar-zinc process has lots of leverage; it turns a fairly small amount of carbon and roughly the same amount of solar heat into two chemical fuels, one of which (carbon monoxide) is good for gas-turbine fuel and some industrial uses, and the other (zinc) which is both portable and can be turned into either electricity or hydrogen with very high efficiency. Another virtue of the process is that it doesn't appear to care where its carbon comes from so long as it is mostly pure (coke, charcoal or even coal) by the time it gets to the input hopper. Within those limits it looks like a great many things will do as carbon sources.

A great many things are made of carbon. The ideal sources would be generated in large quantity, mostly dumped as waste products, and renewable. Biomass may not be the best of fuels, but it does make pretty good charcoal. If we needed enough biomass to get on the order of 240 million tons of carbon per year (per "Going negative"), where could we get it?

This may be a serious underestimate. It appears that some 160 million tons/annum of urban wood waste is uncounted or partially counted in the above (perhaps because it is designated construction waste or yard waste rather than MSW). If even half of this could be captured as biomass, the impact would be very large.

This suggests that something between 120 and 280 million dry tons/year of biomass can come from cities. What else is being thrown away?

Out of the woods

One heck of a lot of wood waste is created in the forest products industry. The national total total is amazing: 178 million mettric tons/year from timber harvesting with 86 million tons unused, and a whopping potential 110 million tons/year from thinning in national forests.

What's the total which could be captured (either currently unused or diverted from their current use)? Heck if I know, but 200 million tons per year seems reasonable.

Off the back 40

Many plants are grown for fruit, seed or tubers but create a great deal of other plant matter as well. In zero-till farming this material can be problematic, as it insulates and prevents the earth from warming as desired in the spring and delays the start of growth. It is desirable to remove this excess matter, but what to do with it?

The stalks and such left over from corn (maize) is called "corn stover". The productivity of corn stover is considerable; at a harvest rate of 170 bushels/acre and allowing 1 ton/acre for ground cover, the remaining matter amounts to 3.0 dry tons/acre. (The 2004 maize harvest was approximately 11.8 billion bu over ~80 million acres, for approximately 150 bu/ac; the corresponding production of surplus stover would be roughly 2.5 dry tons/acre.) Even if corn was reduced from 80 million acres to 60 million, corn stover could provide 150 million tons/year of dry biomass.

What could grow on 20 million idled acres? Switchgrass and Miscanthus have been advanced as biomass crops; they could be planted on buffer zones between fields and waterways to capture nitrogen in runoff and help prevent erosion. Projections of yield are variable, but 10 tons/acre appears reasonable based on some searches. 20 million acres at 10 dry tons/acre would yield 200 million dry tons.

This does not exhaust the list; crops other than maize yield stalks and straw, some of which needs to be burned or otherwise removed to eliminate pests. All of this matter is potential biomass feedstock.

Summing up: 120-280 million tons/year from cities, 200 million tons/year from forests, and 350 million tons/year from current and former maize acreage indicates a potential biomass harvest of 670-830 million dry tons per year. This is sufficient to supply the requirements projected in Going negative. The next question: What should we do with it?

How not to get leverage

Levers can work for you or against you, by either making superior or inferior use of an input in limited supply. One example of a lever which can be disadvantageous is fermentation of carbohydrates to make ethanol. Ethanol's chemical formula is C2H6O; yeasts make it from carbohydrates with a general chemical formula of CH2O, and emit CO2 as a byproduct. If this is the only reaction going on, it balances like this:

3 CH2O -> C2H6O + CO2

One third of the carbon and roughly half the total mass (44 AMU out of 90) is lost as carbon dioxide in the fermentation process. It seems likely that some advocates of ethanol forget this little detail, and it throws their calculations way off. I recall a claim that 300 million tons/year of biomass would create enough ethanol to replace US gasoline consumption. After fermentation this would only yield 153 million tons/year (46.5 billion gallons) of ethanol; this is the energy equivalent of 32.6 billion gallons of gasoline, which is roughly 1/4 of annual US motor gasoline consumption. This claim is clearly false; even without allowing for the smaller energy content of ethanol it would still take upwards of 900 million tons of fermented biomass to replace gasoline, and still more to replace diesel, jet fuel and other uses of petroleum. Replacement of petroleum with ethanol made from near-term renewable biomass stocks is clearly not possible.

Forget ethanol; convert to carbon

If the purpose of the biomass collection is to produce carbon for reduction of metal, it must be pryolized. Pyrolysis produces an off-gas which contains most of the hydrogen and nitrogen and some of the carbon; the maximum recovery achievable under batch conditions using partial combustion for heat is about 30%. It may be possible to increase this yield using external heating rather than partial combustion, but the system would no longer be simple.

The feasible production of carbon from biomass appears to be 210 to 250 million tons per year. This carbon would be dry, sterile and inert, and thus could be stored easily for later use. This carbon could be fed to a thermochemical zinc reduction process, powered either by solar heat or by excess electricity from wind power.

Zinc reduction

If there was 210 to 250 million short tons per year of carbon available, it could be used to produce between 1.14 billion and 1.36 billion tons of metallic zinc per year (from zinc oxide) [1] if the byproduct was carbon monoxide. If the carbon was fully oxidized to CO2 in the reduction process these amounts would be doubled to between 2.28 and 2.72 billion tons of zinc, but any production of power or chemicals from the carbon byproduct would be lost.

Where the rubber meets the road

Using Electric Fuel's figures [2], 1.14-1.36 billion tons of zinc could produce between 966 million and 1.15 billion megawatt-hours (9.66e14 WH to 1.15e15 WH) per year. This is an average power between 110 GW and 132 GW. My previous calculation (somewhat generous) for the amount of power actually delivered to the wheels by vehicles in the USA was around 107 GW average for gasoline vehicles alone and 183 GW including trucks and other diesels; it appears that this amount of zinc could easily replace all gasoline used in the USA, and if we allow for some efficiencies of electric propulsion it could replace the rest of the motor fuel too (give or take a bit). Any extra required could be supplied by regeneration of zinc metal via electrolysis using power from wind, nuclear or any other source of electricity (preferably carbon-free). In the CO-byproduct scenario, an efficiency of 39.8 Wh/mol of CO creates an additional 271-322 632-752 million megawatt-hours (2.71e14-3.22e14 6.32e14-7.52e14 WH) per year, or 72-86 GW of electricity. That's about 32-38% of the amount generated by coal in the USA, or roughly the production from natural gas [3]. (This does not include any energy produced from the pyrolysis off-gas, which may or may not be combustible.)

Conclusion

It appears that a process which uses biomass to produce carbon which is then used to drive a zinc cycle for zinc-air fuel cells could replace all petroleum-based motor fuel used in the USA, and all of the natural gas burned for electric generation as well. No process for turning biomass into ethanol could accomplish anywhere near as much for the same inputs, and no alcohol process can use wind power to generate the same product. Even allowing for rather poor efficiency of zinc-air fuel cells, the zinc route gets much better leverage out of limited inputs.

First, reduction of unrecycled waste. Landfills are a hot political issue and any simple, large-scale process for reducing landfill needs would get immediate attention. In Southern California, R.E.N.E.W. L.A. is a detailed plan to reduce landfill needs by 75-85% through, primarily, gasification and pyrolysis. The political will, environmentalist and public acceptance is growing rapidly, particularly in light of Katrina and gas price inflation.

Second is reducing liquid fossil fuel dependency with renewable fuels. Hybrids, particularly if they run on renewable blends would constitute a stunning shift in the transportation energy paradigm with significant positive impact on air quality. It would certainly bridge the gap between the present and whatever the next paradigm will be (hydrogen fuel cells, for instance).

How do we gradually move business capitalization, infrastructure, and consumer acceptance from here to there? It has to be simple and easy to implement.

Ethanol is key:- Because syngas produced from gasified waste (blended from all the sources you mention) can be converted directly into ethanol with virtually no toxic emissions, while co-generating electricity. Forget sugar fermentation - bacteria can do the job. The concept has been proven in prototype and full-scale plants may start appearing by early 2007.- Because the gasoline infrastructure that exists can already use and easily incorporate marketing ethanol and all blends of gasoline and ethanol.- Because flex-fuel autos in the full-range of consumer-acceptable models exist.- Because markets already exist for ethanol (California imports 99% of the ethanol that constitutes 5.7% of its gasoline).

Progress toward solving either problem would be of vast benefit to our civilization. A single solution that makes progress toward solving both while cogenerating green electricity and reducing toxic emissions seems like a viable way to go.

Reducing landfill needs is all well and good, but this post is all about making the most out of what we're throwing away rather than just being in a hurry to have it gone.

I've not yet seen anything anything regarding the efficiency of the gasification/fermentation schemes. How much of a pound of dry matter at the input becomes useful output? Further, what's the efficiency with which you can utilize that output? (Even the Prius is no great shakes. Elsewhere, I calculated its hypothetical BSFC at a constant 60 MPH and 60 MPG at a non-stellar 0.53 lbm/HP-hr; if gasoline has a HHV of 20,260 BTU/lbm, this works out to a thermal efficiency of 23.7%.)

Once you've converted your stuff to ethanol, you're pretty much stuck with internal-combustion engines as your prime movers. This means you are tied to the inefficiencies - and pollution - they carry with them. The inefficiencies are substantial; a pound of biomass converted with the same efficiency as maize fermentation would yield just 0.31 pounds of ethanol (0.047 gallons, equivalent to just 0.033 gallons of gasoline) which in turn could drive a 35-MPG car just 1.16 miles.

The zinc process could convert 1 pound of biomass to 0.3 pounds of char plus pyrolysis gas (which might be fermentable, but the gas would be a secondary rather tha primary product). That 0.3 pounds can reduce 1.63 pounds of zinc metal, which in turn can produce 690 WH of electricity in a zinc-air fuel cell. Used in a vehicle consuming 300 WH/mile at the cell terminals, it could go 2.3 miles on the energy from that pound of carbon - twice as far as you can manage with ethanol.

Ethanol is the short lever. Zinc is the long one; it has the potential to pry twice as much useful output from a given input.

I am sure EPs estimates of how much biomass can be collected and pyrolsized are quite optimistic in the short term. For instance, I am a forester and know that wood waste (such as slabs and sawdust) is already used for many products, or to provide fuel and heating for sawmills and their processes, while waste left in the woods, in many cases, simply can't pay its way to town.

All that could change, given a market for the materials. The nice thing about the going to the metal reduction process is that we can use good old dirty coal to start out with, then transition to biomass fairly painlessly as infrastructure and markets develop.

The waste doesn't have to pay its own way to town; it could be converted to charcoal on-site (70% weight reduction) and the off-gas used to support logging operations. (I'm not sure how, but an engine fired by the off-gas is bound to be useful for something.)

Now there is an idea. By converting wood waste to charcoal through pyrolisis I assume that your reduce volume a little, but a 70% weight reduction is significant. That might be enough to make this work.

I don't know what you would use the off-gas for, in an active logging operation virtually all of the equipment is mobile and diesel-driven at this time. There is little need for stationary power, except for loaders and, (if slash is being treated) chippers. Slash disposal usually takes place after logging finishes. Perhaps the off-gasses could simply be used to fire a second or third chamber, with no direct burning of wood after the off-gasses start being produced.

The key question would be how small and how portable could the equipment be. Yarding and haul costs are now the big challenges to using a lot of small-diameter and poor quality wood fiber.

I work with private landowners in Utah, which is not a big timber-producing state. I have thousands of acres of aspen stands I help manage that are diseased and choked with submerchantable subalpine fir and other conifers. These stands are in desperate need of renewal, but I can rarely find a market for the timber. At present my best option is to burn the stands. I would much rather see it turned into a useful product. Then there is the issue of fuels reduction in the Pinyon-Juniper zone.....

Heiko Gerhauser was good enough to post a link to a flash carbonization system which is self-powered after ignition; the researcher appears to be in Hawaii, but the system looks like it could be used most anywhere.

Would it be easier to pull the slash out of a stand during logging rather than after? If so, your skidders could do the work for you, and you could feed slash directly to a chipper and thence to a truck-mounted pyrolysis system. The off-gas could perhaps run an engine to power the chipper. Inputs to the pyrolysis process would be slash and air; you might need water to quench the charcoal. A sophisticated system might compress the charcoal into briquettes or other densified forms straight out of the machine.

If the pyrolysis gas is clean enough and has a high enough BTU value it might be worthwhile to co-fuel skidders and other equipment with compressed gas and diesel, but you'd be fueling rather frequently.

A really sophisticated system might feed the excess off-gas to Clostridium cultures to produce ethanol, distill on the spot, and either co-fuel the equipment with ethanol carbureted into the intake or load it onto tankers as a third product. This may be the ne plus ultra but the papers returned by Google Scholar don't seem to mention any working examples even on the laboratory scale.

What about thermal depolymerization (TDP, the "turkey guts" process)? One article said the machinery could be made small enough to fit on a pickup truck. One of the outputs is... carbon. Another output is diesel-like oil. (It also produces a flammable gas, and water.) It is claimed that TDP can handle vegetable matter--it doesn't have to start with oily stuff.

1. It appears to need greasy or fatty inputs (none of the inputs listed in the press kit are high in plant matter).2. The plant in Carthage MO has some extreme odor problems.3. The major output is a liquid fuel better suited to an internal combustion engine (inefficient and potentially polluting) rather than an inexpensive type of fuel cell.

TDP looks like it's best suited for processing animal fats and slaughterhouse waste, vegetable fats and waxes, plastic, rubber and other waste polymers; the lack of thermal (e.g. solar) energy inputs to the conversion process plus the low efficiency of internal combustion engines limits the amount of useful energy you can get out of a given amount of biomass. The biomass pyrolysis / zinc reduction process is better suited to energy production (the solar thermal input yields a substantial boost in the energy yield), and the pyrolysis off-gas might be suitable for fermentation to alcohols, organic acids and ketones with something like Clostridium (though this looks speculative at this time).

"First, reduction of unrecycled waste. Landfills are a hot political issue and any simple, large-scale process for reducing landfill needs would get immediate attention. In Southern California, R.E.N.E.W. L.A. is a detailed plan to reduce landfill needs by 75-85% through, primarily, gasification and pyrolysis. The political will, environmentalist and public acceptance is growing rapidly, particularly in light of Katrina and gas price inflation."

Is this a real issue in the US? From what I gather, very few communities really have a problem with landfills. Even heavily urbanized centers can ship their garbage elsewhere and despite the real estate boom, there is plenty of cheap land out there. Also, most recycling schemes don't seem that useful. For example, why recycle glass when new glass isn't that hard to make?

The only ones as far as I know that are really compelling is aluminum recycling, scrap metal recycling (with the recent increase in metal prices), and toxic materials recycling (eg, lead recycling to avoid regulatory problems). Perhaps paper recycling has reached a useful level, but I still see a premium on paper goods with a high level of recycling.

My point here is that waste disposal came about in the first place because it was cheap and it worked and second because new commodity prices were cheap.

But even in a "peak oil" scenario, I don't see that farm waste will automatically become valuable. We may just find cheaper alternatives than recycling waste.

On my end of things I don't need small diameter and poor quality/unfavored species biomass to become valuable. Just being able to remove significant quantities in a revenue-neutral way would be a huge step forward.

Agriculture may be different. A lot of low-value products such as dry corn stalks and straw can currently be used as maintenance feed for livestock - uses which would compete with biofuel.

Incidentally, I wonder if anyone in this country is looking at this process. One of my local chambers of commerce is all excited because they shipped a container-load of pinyon-juniper material to Ireland for testing with some new enzymes recently developed there for ethanol production. They were given some really optimistic numbers for the amount of fuel that could be generated from a dry ton of chips. I wish I had the numbers they quoted.

Anyway, I wonder if anyone at the Forest Service's Forest Products Lab in Madison, WI has looked into this. They are doing a lot of work with trying to find uses for thinnings and other unmarketable forest products.